This invention is generally related to a personal communications device and, more particularly, to a system and method for determining the ratio between the frequency of two clocks within the personal communications device.
In communications devices such as that described in U.S. Pat. No. 5,945,944 to Krasner et al. for a Method and apparatus for Determining Time For GPS Receivers, the disclosure of which is hereby incorporated herein by reference, it is common for there to be multiple clocks or oscillators that provide clock signals of varying characteristics, including frequency, to device circuitry. In order for the device to operate properly, it is often necessary for certain device operations which operate at different clock frequencies to be synchronized. In order to accomplish this it is necessary to determine the frequency at which a clock operates in relation to a known, or predetermined, reference clock. This is typically done by counting, for a predetermined and known period of time, the number of cycles of a known reference clock having a known frequency, as well as the number of cycles of a second clock of unknown frequency.
A typical set-up for determining a ration between two clock signals is shown in FIG. 1. With reference to FIG. 1, there is provided a numerator latch 1 for receiving and storing the contents of an incrementing counter 7 when a load count signal LCS is received by the numerator latch 1. Incrementing counter 7 is clocked by a clock signal CLK 1. With each pulse of the clock signal CLK 1 the count value of the incrementing counter 7 increases by a value of one (1). There is also provided a denominator latch 4 which, upon receiving load count signal LCS, receives and stores the contents of a incrementing counter 5. Incrementing counter 5 is clocked by a clock signal CLK 2. The value of incrementing counter 5 increases by a value of one (1) with each pulse of the clock signal CLK 2. Upon receiving the load count signal LCS, numerator latch 1 and denominator latch 4 make their respective values available for output as numerator out signal 8 and denominator out signal 9, respectively. In order to compute the ratio of the two clock signals CLK 1 and CLK 2, the values of numerator out signal 8 and denominator out signal 9, respectively. In order to compute the ratio of the two clock signals CLK 1 and CLK 2, the values of numerator out signal 8 and denominator out signal 9 can be divided to produce the ratio between clock signals CLK 1 and CLK 2.
Where, for example, the frequency of CLK 1 is known, the ratio between the value of numerator latch 1 and denominator latch 4 can be used to compute the frequency of the clock signal CLK 2. This process is typically carried out as a part of a dedicated clock pulse count operation and is only as accurate as the resolution of the counting device will allow. These known ratio-counting devices do not provide for dynamically increasing the accuracy of the count while counting of the clock cycles takes place. Thus, a need exists in the industry to address the deficiencies and inadequacies.
This invention is directed to a personal communications device with a ratio counter providing tracking transitioning edges of two clocks so as to generate a signal to initiate capture of a clock cycle count. Provisions for tracking the transitioning edges include a storage memory for storing a first and a second value selected in accordance with the principles of convergents of continued fractions. A first and second counter each responding to first and second clock signal, respectively. The first and second counter each count clock cycles of the respective clock signal. A first register for capturing the count of the first counter and a second register for capturing the count of the second counter.
This invention provides a system and method for calculating a ratio between two clock frequencies in a personal communications device. In architecture, the system may be implemented by a first receiver that includes a first clock generating a first clock signal for clocking the first receiver; a second receiver that includes a second clock generating a second clock signal for clocking the second receiver; and a frequency ratio counter for providing a ratio between the frequency of the first clock signal and the frequency of the second clock signal.
The invention can also be viewed as providing a method for determining a ratio between the frequencies of two clocks. In this regard, the method can be broadly summarized by the following steps: counting successive clock pulses of a fist clock signal for a duration of time determined in accordance with a control signal, counting successive clock pulses of a second clock signal for the duration of time, reading the count of the clock pulses of the first clock signal upon the elapse of the duration; and reading the count of the clock pulses of the second clock signal upon the elapse of the duration. In this method, the control signal is generated where a transitioning pulse edge of the first clock coincides and is in synchronization with a transitioning pulse edge of the second clock.
Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.